The present invention relates to a pneumatically controlled valve of the type which is normally closed, has a constant autocontrolled rate of flow and is particularly but not exclusively used for drying gases by absorption to effect decompression of the installation tanks or reservoirs at a constant rate of flow up to atmospheric pressure, and the successive free venting of the regenerating gas at atmospheric pressure.
Gas drying installations of the adsorption type (see FIG. 1) comprise in general two reservoirs A and B in which there are beds formed by layers of activated alumina and/or silica-gel.
The gas to be dried enters a reservoir A through a three-way valve C properly set and comes out dried through a similar valve D.
A fraction of said dried gas (said fraction being a function of the dimensions of a diaphragm E) flows through a reservoir B regenerating the bed of adsorbing material contained therein and comes out from the installation through a valve arrangement shown at F.
After the regeneration of the bed of reservoir B the bed of reservoir A being exhausted in the meantime, it is necessary to invert operation of the two reservoirs through the valves C and D.
However before effecting the inversion it is necessary to pressurize reservoir B by closing its valve F.
After the inversion reservoir A is brought back to atmospheric pressure by opening its valve F which should theoretically effect a slow decompression under constant rate of flow and pressure gradient.
In general normally closed valves are used at F. Said known valves do not permit a gradual opening securing a slow and regular decompression of the reservoir previously operating in the drier mode so causing frequently lifting and upsetting of the adsorbing bed.
Further during the successive regeneration step, which occurs at atmospheric pressure the same valves must secure an unobstructed discharge of the gas employed for the regeneration. To achieve this a passage sectional area is needed which is from 5 to 10 times greater than the optimal one used in the decompression step.
In general the mentioned problems are only partially solved by using two valves instead of the only valve of the invention; one of said two valves is a decompression valve having a passage sectional area adequately small and the second one of greater size working sequentially to, and mounted separately from the first mentioned valve. The second valve permits the unobstructed discharge at atmospheric pressure of the regenerating gas, however this involves higher costs and a more complex operation of the programmer device controlling the valves.
It has been further ascertained that the decompression step, although being conducted with a valve having an adequately small passage sectional area does not have--in opposition to what is desired--a linear characteristic of the function time us pressure variation but only an exponential one.
Instead of two valves it has been suggested to use a single valve of proper size, the valve member of which is controlled by a big pneumatic servo-motor, in general of the membrane type, operated by the pressure differential produced by a diaphram or Venturi tube in the exhaust conduit downstream the valve. Said known valve obtains a decompression with a linear gradient, but, by employing a downstream located meeasurement orifice or Venturi tube, it causes a resistance, i.e., a backpressure on the regenerating gas, and thus an efficiency reduction of the regeneration.